test(e2e): use host machine instead of Docker
The Docker E2E test aren't hermetic, and we've been having issues getting them to run on CI (see !23 (closed)). We could switch to using QEMU for everything, but that would be very slow, given it takes around 100-120s for the image to boot.
However, we can trick Labgrid into treating the host machine as a target board for running most E2E tests.